The invention relates to a coextruded, biaxially oriented opaque film having a low density and composed of a base layer which essentially consists of polypropylene and fillers, and surface layers present on one or both sides. The invention further relates to a process for the manufacture of these films, and to their use.
A film of this nature is known from U.S. Pat. No. 4,303,708. According to the teaching of this patent, the film is supposed preferably to contain 5 to 15% by weight, and according to the example only 8% by weight, of filler particles. Also it is supposedly not critical, in the manufacture of the film, whether the longitudinal stretching and transverse stretching are carried out successively or simultaneously. The stretch ratios are also not considered important and are, according to the example, 5.5 (in the longitudinal direction) and 9.0 (in the transverse direction).
The film produced according to the example of the above patent has a relatively low density of 0.9 g/cm.sup.3, attributable to a large number of cavities (vacuoles) which are formed on stretching the coextruded film.
In fact, during stretching, the polymer matrix is torn open at the grain boundaries of the organic or inorganic fillers which are incompatible with the polypropylene, leading to the formation of the said vacuoles in the film, i.e. to free, empty, unfilled spaces in the film. It is self-evident that these vacuoles or micro-cavities are distributed over the entire volume of the film. Due to the refraction (scattering) of light at these cavities, the film has an opaque surface with a gloss resembling mother of pearl.
Because of the opaque or mother of pearl appearance, these films have a promotionally extremely effective appearance. Since they are also readily printable, they are employed for diverse purposes, especially as a packaging film, among which, above all, chocolate bar packaging should be mentioned. Broader use of the known coextruded and biaxially stretched opaque polypropylene multi-layer film has not been made, however, due, in particular, to its relatively high cost and due to the fact also that it still requires improvement in various mechanical properties. These include its foldability, especially its pronounced tendency to revert, after bending, folding or creasing, to its starting position before folding.
This disadvantage is in principle exhibited by all stretch-oriented polypropylene films, even if they do not contain any filters. This property is particularly undesirable if the film is envisaged as a packaging film for wrapping relatively small articles of substantially round cross-section such as, for example, candles, bottles, candles, rolls of fruit drops or chocolate bars. In this process, the article is first completely wrapped in a rectangular piece of film and the closure is formed by twisting together the two projecting film ends. Biaxially oriented polypropylene films in this application show the disadvantage that their twisted film ends execute a reverse movement counter to the twisting direction, so that there is then the danger that the packaging will open.
A further prerequisite for twisting the film during packaging is the property of the film that it will not start to tear, or tear off completely, at the twist points but on the other hand is sufficiently rigid that during twisting it ill not shrink or crumple. This property, again, is insufficiently displayed by the known biaxially oriented polypropylene films, which is why, for example, regenerated cellulose film is predominantly employed for candy wrappings.